1. Field of the Invention
The present invention relates to a worm speed reducer.
2. Description of Related Art
In an electric power steering system, rotation output from an electric motor is transmitted to a worm shaft of a worm speed reducer. The speed of the rotation output from the electric motor is reduced with a worm wheel that meshes with the worm shaft. The resultant rotation is transmitted to a steering operation mechanism to assist generating a torque needed for a steering operation. Backlash is needed to allow the worm shaft and the worm wheel to mesh with each other. However, tooth hammering noise (rattling noise) may be caused by the backlash during traveling.
Thus, an electric power steering system has been proposed, in which the worm shaft is supported such that a first end of the worm shaft pivots around a second end of the worm shaft and the first end is elastically biased toward the worm wheel by a bias member, thereby eliminating the backlash based on the above-described configuration. Typically, the worm shaft is configured like a right-hand thread. Only when the worm shaft, configured like a right-hand thread, rotates clockwise, a meshing reaction force (driving reaction force) that the worm shaft receives from the worm wheel in a meshing area induces a moment that biases a meshing portion of the worm shaft toward the worm wheel (the same direction as the biasing direction of the bias member) around the second end of the worm shaft.
Therefore, meshing frictional resistance generated when the worm shaft rotates clockwise is larger than meshing frictional resistance generated when the worm shaft rotates counterclockwise. Thus, frictional resistance torque generated in the former case is higher than frictional resistance torque generated in the latter case. As a result, steering feeling may vary according to a steering direction. On the other hand, techniques for the electric power steering system have been proposed: the technique in which a pair of tooth flanks of the worm shaft has different pressure angles and the technique in which a pair of tooth flanks of each tooth of the worm wheel has different pressure angles (see, For example, Japanese Patent Application Publication No. 2006-103395 (JP 2006-103395 A) (paragraph [0041] and FIG. 5)).
The technique in JP 2006-103395 A (paragraph [0041] and FIG. 5) is expected to provide an effect that suppresses a difference in frictional resistance torque due to a difference in rotating direction.
However, for the worm shaft and the worm wheel, a cutting tool with a special shape is needed to provide a pair of tooth flanks with different pressure angles. That is, a plurality of cutting tools is needed which has different shapes depending on the specification of the worm shaft or the worm wheel, resulting in increased manufacturing costs of the worm speed reducer as a whole.